The Future of Proposer-Builder Separation in a Multi-Layer World

PBS is the new consensus. The primary role of a base layer shifts from execution to ordering and settlement, making proposer-builder separation the fundamental protocol for coordinating value flow across rollups, appchains, and L2s.

Monolithic L1s are inefficient. A single chain cannot optimize for security, scalability, and sovereignty simultaneously. Specialized layers like Arbitrum for execution, Celestia for data availability, and EigenLayer for security prove modular architectures win.

Builders become cross-chain orchestrators. Entities like Flashbots and bloXroute will evolve from block builders to cross-domain sequencers, optimizing MEV extraction and transaction ordering across Ethereum, Arbitrum, and Optimism in a single bundle.

Evidence: Over 90% of Ethereum's value is now settled on L2s. Arbitrum and Optimism collectively process more transactions than Ethereum L1, proving demand has irreversibly shifted to the execution layer.

Single-chain PBS is insufficient for a modular stack. Isolated PBS on Ethereum, Arbitrum, and Solana creates fragmented liquidity and cross-domain MEV leakage. Builders cannot coordinate execution across layers, forcing users into inefficient, high-slippage routes.

Cross-layer PBS is a coordination primitive. A unified PBS layer enables builders to atomically settle bundles across Ethereum, Optimism, and Polygon zkEVM. This turns interop protocols like Across and LayerZero into execution venues, not just message bridges.

The alternative is extractive fragmentation. Without it, vertical integration wins. Rollup sequencers like Arbitrum and zkSync will internalize cross-chain MEV, creating walled gardens of value capture that undermine the open, composable ecosystem.

Evidence: UniswapX and CowSwap already demonstrate intent-based, cross-domain settlement. A generalized PBS layer formalizes this, allowing builders to compete on sourcing liquidity from Celestia DA or EigenLayer AVS data, not just local mempools.

Cross-domain bundles are the new primitive. They are single, signed payloads containing instructions for multiple, heterogeneous chains, executed atomically by a unified sequencer network. This replaces the manual, error-prone process of managing separate transactions across Ethereum, Arbitrum, and Solana.

The bundle is the product, not the transaction. Builders compete to construct optimal bundles that satisfy complex user intents, like cross-DEX arbitrage or leveraged position management across Aave and dYdX. This shifts the economic center of gravity from simple block building to cross-domain state optimization.

PBS evolves into Cross-Domain PBS (CDPBS). A new role, the Cross-Domain Builder, emerges. They must source liquidity and compute across domains, using systems like EigenLayer for restaking security and Succinct for proof aggregation, to guarantee atomicity. The winning bundle is a single, verifiable claim of multi-chain state change.

Evidence: MEV already spans domains. Flashbots' SUAVE and protocols like Across demonstrate the demand for cross-chain intent fulfillment. The next evolution bundles these actions into a single, auctionable unit for Ethereum proposers and Alt-L1 validators to sell.

Single-domain PBS is obsolete. Isolated PBS per chain (e.g., Ethereum, Solana) creates fragmented markets, allowing builders to extract monopoly rents on each domain. The future is a cross-domain PBS network that aggregates blockspace across rollups and L1s into a single auction.

Shared sequencing is the first step. Rollups like Arbitrum and Optimism outsourcing to a shared sequencer set (e.g., Espresso, Astria) create the atomic composability prerequisite for cross-domain PBS. This shared sequencing layer becomes the natural substrate for a unified builder market.

Intent-based primitives are the execution engine. Builders will not manually construct cross-chain bundles. They will fulfill user intents expressed through systems like UniswapX or Across, using solvers and bridges like Stargate to atomically execute across domains. The builder's role shifts from manual assembly to intent resolution.

Credible neutrality requires open protocols. A proprietary cross-domain PBS controlled by a single entity (e.g., a foundation or L2 team) recreates the centralization problem. The solution is a permissionless builder marketplace with standardized APIs, akin to how Flashbots' SUAVE envisions a decentralized block-building network.

Evidence: The economic incentive is undeniable. A builder capturing MEV across Ethereum and ten major rollups today must win ten separate auctions. A unified auction consolidates this power, increasing efficiency but demanding robust, neutral infrastructure to prevent a single-point cartel.